Charles and Camilla's limousine had just turned into Regent Street on their way to the Royal Variety Performance when around 20 demonstrators threw paint and bottles over the car and shattered a window.

While visibly shaken, the couple were left "unharmed" and continued on to their engagement," a Clarence House spokesperson said.

The incident took place as the latest demonstrations against a raise in tuition fees descended into violence and vandalism.

Benches were set fire to in Parliament Square and windows smashed at the Treasury as protestors made their way down Oxford Street.

With questions raised about royal security, Prime Minister David Cameron has condemned the attack as "shocking and regrettable".